MadSci Network: Evolution |
Brian, An interesting question, and one which exercised Darwin in his time. His colleagues had noticed that remote islands often had populations of plants and animals which were apparently unable to cross the oceans, and one of the theories current at that time was that there had been a huge continent which had become submerged, except for the islands, and that was why the fauna and flora seemed to be similar to nearby continents. When Darwin suggested they had been carried there by birds or ocean currents he was ridiculed; people said that, for instance, seeds of land plants would be killed by salt water. Darwin filled a tank with seawater, put some seeds from his garden in it, and some of them actually germinated better after several weeks in salt water ! He also suggested that seeds would be carried on bird's feet, and washed dirt from the feet of wild birds shot by sportsmen - seeds germinated from it. A new volcanic island called Surtsey appeared off Iceland, on the mid-Atlantic ridge, in 1963. There are lots of websites about it - try http://www.geophys.washington.edu/People/Students/throstho/Vinna/surtsey/ nytt_surtsey.html Already there are around 50 species of plants established there and birds are breeding. There are also invertebrate animals - apparently spiders were among the first animals to arrive. Newly hatched spiders spin long threads of silk which enable them to drift in the atmosphere, so their presence is no surprise. A small sparrow-like bird has begun to breed there, the snow bunting, showing that there must now be abundant insect life available. It is true that such small birds don't fly very far when collecting food, but most birds are migratory, and the enormous distances covered by the smallest birds prove that any species could easily reach the Galapagos. Look in any popular bird book for winter and summer ranges of birds. Darwin's ideas on speciation concern what happens to the birds after they choose to remain isolated on the island, not how they got there. He didn't suggest that they appeared there, he observed that they were similar to, but different from species on the mainland, inferring that the mainland birds must be their ancestors, and isolation in a new environment over many generations led to evolutionary change. The only mammals on the Galapagos are seals and sealions, but you are right to point out that the existence of reptiles there is problematical. These are the only species there which require a far-fetched explanation, and I can't think of a better one ! The fact that these reptiles are quite different from mainland species means that they have been there for a very long time, in order to evolve new varieties. This is one reason why it is unlikely that they were brought there by humans. Most people think humans have only been in the Americas for 10 to 20 thousand years. Some anthropologists are beginning to take more interest Thor Heyerdal's ideas about ancient peoples travelling across the Pacific from Polynesia, but this only gives us another couple of tens of thousands of years at most. One other thing to remember - we aren't necessarily talking about tortoises balancing on treetrunks. In tropical regions, large mats of vegetation can be washed out to sea, certainly carrying animals. Hope this helps - email me if you want to discuss further.
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